The use of a brake booster in the braking system of a motor vehicle is well known. The brake booster typically comprises a front and rear housing parts which are joined together around an outer peripheral edge; a flexible diaphragm secured inside the housing to define front and rear chambers within the housing; and a piston movable along its axis relative to the housing. At one or more positions within the booster, a cylindrical component of the booster has to pass though a seal during assembly of the booster; for example, the piston passing through an internally mounted seal, or an air passage tube extending through a sealing aperture in the diaphragm. In general, such a cylindrical component has an external diameter which is greater than the internal diameter of the seal prior to assembly. This can make installation of the cylindrical component through the seal difficult, especially where the assembler is unable to clearly see the installation during assembly (that is, during blind installation).